Question

SELECT 
    *
FROM 
    myTable
WHERE 
    field1 LIKE 'match0' AND
    myfunc(t1.hardwareConfig) LIKE 'match1'

Here is my question,

the matching of field1 is fast and quick, but myfunc takes forever to return and I want to make sure that if field1 doesn’t match that it doesn’t even attempt to do myfunc.

Will SQL just know this or can I make it explicit in my query?

I’m on MSSQL 2000, 2005 and 2008, hopefully there’s a common answer.

Answer 1

To enforce the order in which conditions are evaluated, use the following approach, because it is documented that CASE preserves the order in which conditions are evaluated.

SELECT 
        *
FROM 
        myTable
WHERE 
  CASE WHEN field1 LIKE 'match0' THEN 
    CASE WHEN myfunc(t1.hardwareConfig) LIKE 'match1' 
      THEN 1 
    END 
  END = 1

The following article explains it in good detail: Predicates in SQL